Historic Dispute : Do neutrinos have mass?

Viewpoint:
Yes, the Japanese-U.S. research team called the Super-Kamiokande
Collaboration announced experiment results in 1998 that proved that
neutrinos do indeed have mass.

Viewpoint:
No, the experiments of earlier twentieth-century scientists repeatedly
indicated that neutrinos did not have mass.

In 1931 Wolfgang Pauli first predicted the existence of a subatomic
particle that Enrico Fermi would later name the neutrino. The neutrino
must exist, Pauli reasoned, because otherwise the atomic process known as
beta decay would violate the physical laws of conservation of energy and
conservation of angular momentum. Neutrinos had never been detected, so
Pauli concluded that they didn't interact with most other particles
or forces. This implied they were extremely small particles with no charge
or mass.

The only force that noticeably affects neutrinos is the
"weak" force, a subatomic force that is not as strong as the
force that holds the atomic nucleus together, but that likewise operates
only at very short range. Because of their extremely limited interactions
with other particles and forces, neutrinos can travel huge distances
unimpeded by anything in their path. Neutrinos arising from the nuclear
reactions in the Sun stream through Earth and all its inhabitants without
having any effect whatsoever. For this reason they are extremely difficult
to detect, and their existence was not confirmed until a quarter century
after Pauli's prediction.

Today's neutrino detectors, kept deep underground to avoid stray
particles on Earth's surface, may contain thousands of tons of
fluid. While trillions of neutrinos pass through the fluid every day, only
a few dozen are likely to be detected.

Scientists have discovered that there are three types of neutrinos, each
associated with a different charged particle for which it is named. Thus
they are called the electron neutrino, muon neutrino, and tau neutrino.
The first type of neutrino to be discovered was the electron neutrino, in
1959. The muon neutrino was discovered in 1962. The tau neutrino has yet
to be directly observed. It was inferred from the existence of the tau
particle itself, which was discovered in 1978. The tau particle is
involved in decay reactions with the same imbalance that Pauli solved for
beta decay by postulating the electron neutrino.

One ongoing issue in neutrino research is called the "solar
neutrino problem." This refers to the detection of fewer electron
neutrinos than expected, given the known energy output of the Sun. One
possible explanation for this phenomenon could be
"oscillation" between the different neutrino types. That is,
electron neutrinos could change into muon or tau neutrinos, which are even
more difficult to detect. Similarly, scientists have observed a deficit in
the number of muon neutrinos they would expect to see coming from cosmic
rays.

Neutrino oscillations, if they exist, are a quantum mechanical phenomenon
dependent on the difference in the masses of the two types of particles.
That means that if researchers could prove that neutrino oscillation
occurs, at least one of the neutrino types involved must have a non-zero
mass. In 1998 a Japanese-U.S. research team called the Super-Kamiokande
Collaboration announced that they had discovered evidence of oscillation
between muon neutrinos and either the tau neutrino or a new, unknown type.

—SHERRI CHASIN CALVO

Viewpoint: Yes, the Japanese-U.S. research team called the
Super-Kamiokande Collaboration announced experiment results in 1998 that
proved that neutrinos do indeed have mass.

Wolfgang Pauli inferred the existence of the neutrino in 1930 from the
discovery that a small amount of energy was missing in the decay
products in certain types of radioactivity (beta decay). In this type of
decay, a neutron in an atomic nucleus is converted into a proton, with
emission of an electron and an antineutrino, a neutral particle that
could not be detected directly. Pauli also suspected that the mass of
this particle would be zero because it could not be detected in the
decay experiments. The mass difference between the initial and final
nuclei produced during this decay should correspond to the highest
energy of the emitted particles. Any difference would then correspond to
the mass of the neutrino itself, according to Einstein's
mass-energy relationship. This energy difference, however, proved too
small to be measured.

Neutrinos escaped direct detection until 1956 when Frederick Reines and
Clyde Cowan Jr. captured neutrinos produced in a nuclear reactor. In
subsequent experiments, the speed of neutrinos could not be
distinguished from that of light. Also, there was no spread in the
arrival time of neutrinos produced by the supernova SN observed in 1987.
According to relativity, particles such as photons that travel at the
speed of light have zero masses. This reinforced the idea that neutrinos
might have zero mass.

The first hint that something might be wrong with this picture came in
1967 when scientists tried to detect neutrinos created by nuclear
reactions in the sun's core. In this and subsequent experiments,
scientists consistently observed a shortfall of about half of the
neutrinos that were predicted by theory.

However, already in 1957, long before the detection of solar neutrinos
proved to be a problem, the Italian physicist Bruno Pontecorvo, then
working in the Soviet Union, proposed that neutrinos could change
continuously from one type into another. Now we know that three types,
or "flavors," of neutrinos exist: tau, muon, and electron
neutrinos, named after the particles they are associated with during
their creation. The continual change from one type into another type of
neutrino is called "oscillation." This phenomenon is a
quantum mechanical effect (superposition of several quantum states) and
is also observed in other elementary particles, such as K0 mesons. For
example, if we detect a muon neutrino, it is possible that it just
oscillated from being an electron neutrino before arriving at the
detector. But according to quantum mechanics, oscillations are only
permitted if neutrinos have mass.

The Standard Model of particle physics, a global theory that describes
elementary particles and their interactions, developed since the 1970s,
predicted neutrinos with zero mass. Therefore, at first the fact that
neutrinos may have mass was viewed as a possible crack in the Standard
Model. But theorists are now extending the Standard Model so it can
incorporate neutrino masses.

The Atmospheric Neutrino Anomaly

The neutrino deficit shown by the solar neutrino experiments was the
first indication that the Standard Model did not describe these
particles accurately: some changed flavor and thus escaped detection.
Another so-called neutrino anomaly became apparent around the mid-1980s.
Both the Kamiokande detector in the Japanese Alps and the
Irvine-Michigan-Brookhaven (IMB) detector in the Morton salt mine in
Ohio started observing neutrinos produced by cosmic-ray particles
colliding with atomic nuclei in the earth's atmosphere. According
to theory, these interactions should produce twice as many muon
neutrinos as electron neutrinos, but the detectors discovered roughly
equal amounts. Scientists explained this discrepancy by the possibility
that muon neutrinos change into either electron or tau neutrinos before
reaching the detectors.

The successor to the Kamiokande is the Super-Kamiokande, the
world's largest neutrino detector. It consists of a huge tank
containing 50,000 tons of purified water, and it is protected from
cosmic rays by a 1,000-m (c. 1,094 yd) rock layer. Eleven thousand
photomultiplier tubes line the tank and track the light flashes
(Cerenkov radiation) caused by muons or electrons
produced by neutrinos interacting with nuclei that travel through the
water at velocities higher than the velocity of light in water.

The photodetectors also allow the tracking of the direction of the
incoming neutrinos. In 1998 Super-Kamiokande results showed that about
half of the muon neutrinos produced in the earth's atmosphere
that had traveled through the earth over a distance of 13,000 km (c.
8,078 mi) had disappeared, as compared to those produced in the
atmosphere just overhead of the detector.

Accelerator-Based Experiments

One of the weaker points in the neutrino experiments with solar or
atmospheric neutrinos is the impossibility of controlling the neutrino
source: there was always the possibility that theory simply predicted
wrong neutrino fluxes. Therefore researchers tried to use neutrinos
created in nuclear reactors or particle accelerators. At CERN, the
European Laboratory for Nuclear Physics, a proton beam from the Super
Proton Synchrotron (SPS) was aimed at a beryllium target for the
production of muon neutrinos.

These muon neutrinos were aimed at two detectors, NOMAD and CHORUS,
placed at a 900-m (c. 984 yd) distance from the neutrino source that
tried to pick up tau neutrinos that would have been formed by the muon
to tau oscillation. This type of search is called an
"appearance" search, and an appearance of a different type
of neutrino than those that created in the accelerator would be a much
stronger proof that neutrinos oscillate than a
"disappearance" experiment. But data taken with both
detectors have not revealed a measurable neutrino oscillation. Several
similar experiments, using nuclear reactors for neutrino sources, such
as CHOOZ in the French Ardennes, also could not confirm neutrino
oscillations.

In 1996 researchers using the Liquid Scintillating Neutrino Detector
(LSND) at Los Alamos announced the detection of electron antineutrinos
produced by oscillating muon antineutrinos. The muon antineutrinos were
produced by a proton beam hitting a target in an accelerator. However,
some researchers doubt the results because the obtained oscillation
rates differ from oscillation rates measured with other experiments.
Others believe that a fourth kind of neutrino, a so-called sterile
neutrino that only interacts very little with matter, may explain the
discrepancy in results.

Long Baseline Experiments

A new generation of long baseline experiments will allow physicists to
pin down neutrino oscillations much more accurately. Because the
neutrinos travel over much longer distances, it is more likely they
would undergo oscillations. A first successful experiment, dubbed K2K,
was announced in Japan in July 2000. Neutrinos produced in an
accelerator at the Japanese National Accelerator Facility (KEK) in
Tsukuba were aimed at the Super-Kamiokande. The neutrinos traveled over
a distance of 250 km (155 mi). During an experimental run that lasted
from June 1999 to June 2000, the Super-Kamiokande only detected 27 muon
neutrinos from the 40 muon neutrinos it would have detected if neutrinos
would not oscillate. In July 2001 K2K researchers announced that a total
of
44 muon neutrinos have been detected from 64 that should have arrived
if neutrinos did not oscillate.

The U.S. Department of Energy's Fermilab near Chicago is now
planning to set up a long baseline terrestrial experiment called MINOS
(Main Injector Neutrino Oscillation). Muon neutrinos, produced by a
proton accelerator at Fermilab, will be aimed at a detector placed 800 m
(c. 875 yd) underground in the Soudan iron mine in Minnesota at a
distance of 730 km (438 mi) from Fermilab. Neutrinos will be detected by
electronic particle detectors placed in between stacks of steel plates.
The energy range of the neutrinos will be similar to the atmospheric
neutrinos detected by Super-Kamiokande, and therefore MINOS would be an
independent check on the Super-Kamiokande results.

In Europe, a long baseline experiment is planned for 2005, when CERN
will aim pulses of neutrinos at the Gran Sasso National Laboratory, the
world's largest underground laboratory excavated deep under a
mountain in the Appenines in central Italy. At CERN, a proton beam from
the SPS (Super Proton Synchrotron) will hit a graphite target and
produce muon neutrinos. While traveling over a distance of 730 km (438
mi) to Gran Sasso, some of the muon neutrinos will change into tau
neutrinos. Two detectors, OPERA and ICANOE, will look for these tau
neutrinos. Unlike the other long baseline experiments, which look for
the disappearance of certain types of neutrinos, the CERN-Gran Sasso
experiment will look for the appearance of neutrinos that have undergone
oscillations, and therefore the results will be much more reliable.

Longer baselines will also allow a higher accuracy, and scientists are
already studying the possibilities of building "neutrino
factories," muon storage rings that would produce very intense
beams of neutrinos that could be aimed at detectors several thousands of
kilometers away.

—ALEXANDER HELLEMANS

Viewpoint: No, the experiments of earlier twentieth-century scientists
repeatedly indicated that neutrinos did not have mass.

From stone circles arranged by the ancient Celts to space-based
telescopes, astronomers have always kept their eyes on the
skies—unless they are looking for neutrinos, not galaxies or
planets, but subatomic particles that have had scientists pounding the
theoretical pavement since 1931.

It all started around 1900 when a series of experiments and some good
luck showed physicists that the atom was not a featureless ball. Rather,
it had an internal structure. The atomic age was born. It was radiation
that got their attention—atoms of elements like uranium emitted
some kind of ray.

Discovery of Neutrinos

By the 1920s they knew the atom had a core, which they named a nucleus,
with electrons moving around it. Inside the nucleus were protons and
neutrons. Some of the nuclear radiation turned out to be beta particles
(speeding electrons) that the nucleus emitted when heavier elements
decayed into lighter ones. But atoms that emitted beta particles did so
with less energy than expected. Somehow energy was being destroyed, and
it was starting to look like physicists might have to abandon the law of
conservation of energy.

Then, in 1931, physicist Wolfgang Pauli suggested that when an atom
emits a beta particle it also emits another small particle—one
without a charge and maybe without mass—that carries off the
missing energy.

In 1932 physicist Enrico Fermi developed a comprehensive theory of
radioactive decays that included Pauli's hypothetical particle.
Fermi called it a
neutrino,
Italian for "little neutral one." With the neutrino,
Fermi's theory explained a number of experimentally observed
results. But it took another 25 years or so to prove neutrinos existed.

American physicists Frederick Reines and Clyde Cowan Jr. conducted an
elaborate experiment in 1956 at the Savannah River nuclear reactor. They
set up a detection system that focused on one reaction a neutrino might
cause and detected the resulting gamma rays produced at just the right
energies and time intervals. In 1959 they announced their results, and
they later shared the 1995 Nobel Prize in physics for their contribution
to the discovery. This neutrino was later determined to be an electron
neutrino. Their findings confirmed Pauli's theory, but that was
not the end of questions about neutrinos.

Further Experiments

In 1961 physical chemist Ray Davis and theoretical physicist John
Bahcall started wondering if there was a direct way to test the theory
of how stars shine. They wanted to find a way to observe neutrinos that
were supposed to be produced deep inside the Sun as hydrogen burned to
helium. They knew it was a long shot, because anything that could escape
from the center of the Sun would be very hard to detect with a
reasonable-sized experiment on Earth.

The next year, in 1962, experiments at Brookhaven National Laboratory
and CERN, the European Laboratory for Nuclear Physics, discovered that
neutrinos produced in association with particles called muons did not
behave like those produced in association with electrons. They had
discovered a second neutrino flavor, the muon neutrino.

Two years later, in 1964, Bahcall and Davis proposed the feasibility of
measuring neutrinos from the Sun, and the next year—in a gold
mine in South Africa—Reines and colleagues observed the first
natural neutrinos, and so did researchers Menon and colleagues in India.

In 1968 Ray Davis and colleagues started the first radiochemical solar
neutrino experiment using 100,000 gallons of cleaning
fluid—carbon tetrachloride—a mile underground in the
Homestake gold mine in South Dakota. The chlorine in the cleaning fluid
absorbed neutrinos and changed chlorine atoms into detectable
radioactive argon. But the Homestake experiment captured two or three
times fewer solar neutrino interactions than Bahcall calculated on the
basis of standard particle and solar physics. Physicists called it the
solar neutrino problem.

Several theorists suggested the missing electron neutrinos had
oscillated—turned into another kind of neutrino that the
Homestake experiment could not detect—but most physicists thought
it was more likely that the solar model used to calculate the expected
number of neutrinos was flawed. Besides, neutrino oscillation would be
possible only if neutrinos had mass, and current theories said neutrinos
had no mass.

Debate over Neutrino Mass

But the discrepancy between standard calculation and experimental
detection was telling physicists about something new in particle
physics—that something unexpected happened to neutrinos after
they were created deep inside the Sun.

In 1985 a Russian team reported measuring neutrino mass—10,000
times lighter than an electron's mass—but no one else
managed to reproduce the measurement.

The Kamiokande detector went online in 1986 in the Kamioka Mozumi mine,
186 miles northwest of Tokyo in the Japanese Alps. For 1,200 years the
mine had given up silver, then lead and zinc. Now, down among its 620
miles of tunnels, was a plastic tank, bigger than an Olympic swimming
pool, full of ultraclean water and thousands of photomultiplier tubes
that detected tiny flashes of light in the dark mine
water—spontaneous proton decay. When a tube detected a flash it
sent an electronic signal to a computer that recorded time and location.

There were no flashes at Kamiokande until February 23, 1987, at 7:35 A .
M ., Greenwich mean time. That morning, the Kamioka detector recorded 11
events, each made up of 30 photo-multiplier flashes in a certain time
sequence and pattern. Something had entered the detector and interacted.
It all took 10 seconds.

When a massive star explodes as a supernova, the blast unleashes a
thousand times more neutrinos than the Sun will produce in its
10-billion-year lifetime. More than 20 years earlier, theoretical
astrophysicists said supernovas should release huge numbers of
neutrinos. They had come from the core of the exploding star, escaping
into space. Hours or days later, a shock wave from the main explosion
would reach the surface, producing a blast of light as the star blew
apart. And that is what happened at Kamiokande.

Several hours after the neutrinos hit the Kamioka mine, Supernova 1987A
became the first exploding star in 384 years to be seen by the naked
eye. Two years later, in 1989, Kamiokande became the second experiment
to detect neutrinos from the Sun and confirmed the long-standing solar
neutrino problem—finding about a third of the expected neutrinos.

Exploring Oscillation

For years there were experimental hints for neutrino oscillation, mainly
from the smaller than expected number of solar electron neutrinos. Other
experiments hinted at oscillations by muon neutrinos produced in the
upper atmosphere, in a decay chain that yielded two muon neutrinos for
every electron neutrino.

But early experiments at Kamiokande and at the
Irvine-Michigan-Brookhaven detector near Cleveland suggested the
muon-to-electron-neutrino ratio was one, not two. If that was true, half
the muon neutrinos were missing. Physicists needed proof to show the
cause was neutrino oscillation. To answer that question, in 1996 another
detector went online at the Kamioka mine. Super-Kamiokande was a $130
million neutrino detector built to find out whether neutrinos had mass.

This detector is a big tank of clean water 1 km (0.6 mi)
underground—a 50,000-ton cylinder, 132 ft around and high, with
11,146 photomultiplier tubes lining the tank. The tubes are each
sensitive to illumination by a single photon of light—a level
about equal to the light visible on Earth from a candle at the distance
of the Moon.

Any charged particle moving near the speed of light in water produces a
blue Cerenkov light, sort of a cone-shaped optical shock wave. When an
incoming neutrino collides with an electron, blue light hits the
detector wall as a ring of light. A photomultiplier tube sees the light
and amplifies it, measuring how much arrived and when, and the computer
registers a neutrino hit.

The tube array also samples the projection of the distinctive ring
pattern to determine a particle's direction. Details of the ring
pattern—especially whether it has a muon's sharp edges or
an electron's fuzzy, blurred edges—is used to identify
muon-neutrino and electron-neutrino interactions.

In 1997 Super-Kamiokande reported a deficit of cosmic-ray muon neutrinos
and solar electron neutrinos at rates that agreed with measurements by
earlier experiments. And in 1998, after analyzing 535 days of data, the
Super-Kamiokande team reported finding oscillations—and so
mass—in muon neutrinos.

Scientists Explore Neutrino Properties

This was strong evidence that electron neutrinos turned into muon and
tau neutrinos as they streamed away from the Sun, but astrophysicists
needed something more. The Sudbury Neutrino Observatory (SNO), 2 km (1.2
mi) underground in INCO's Creighton mine near Sud-bury, Ontario,
went online in November 1999 to determine whether solar neutrinos
oscillate on their trip from the Sun's core to Earth and to
answer other questions about neutrino properties and solar energy
generation.

The SNO detector is the size of a 10-story building. Its 12-m-diameter
(c. 13.1 yd) plastic tank contains 1,000 tons of ultrapure heavy water
and is surrounded by ultrapure ordinary water in a giant 22-m-diameter
by 34-m-high cavity. Outside the tank is a 17-m-diameter geodesic sphere
that holds nearly 9,500 photomultiplier tubes that detect light flashes
emitted as neutrinos stop or scatter in the heavy water.

The detector measures neutrinos from the Sun in two ways—one
spots a neutrino as it bounces off an electron (any of the three
neutrino flavors cause a recoil and are detected); the other detects an
electron neutrino when it hits a neutron in SNO's 1000-ton sphere
of heavy water. Only an electron neutrino can make the neutron emit an
electron and trigger the detector. The two methods, along with results
from Super-Kamiokande, were designed to show how many neutrinos come
from the Sun and what proportion are muon or tau neutrinos.

On June 18, 2001, SNO's Canadian, American, and British
scientific team announced they had spotted neutrinos that had been
missing for 30 years. SNO confirmed what several experiments, especially
Super-Kamiokande in Japan, had already shown—the missing electron
neutrinos from the Sun had changed to muon and tau neutrinos and escaped
detection.

The transformation also confirmed earlier observations that neutrinos
had mass, and the SNO measurements agreed with first-principles
calculations of the number of solar neutrinos created by the Sun. The
solar neutrino problem was solved, according to a team member, with a
99% confidence level. The answer is oscillations.

The Standard Model

But does the solution to the solar neutrino problem create more problems
for the Standard Model of particle physics? Neutrinos are massless in
the Standard Model, but the model could be extended to include massive
neutrinos through the Higgs mechanism—a phenomenon that
physicists believe gives other particles mass.

But most particle theorists do not want to extend the model. They prefer
using another version of the Higgs mechanism called the seesaw
mechanism. This includes neutrino interactions with a very massive
hypothetical particle. For the range of parameters indicated by data
from Super-Kamiokande, the heavy mass would be within a few orders of
magnitude of the scale where physicists believe strong and electroweak
forces unify.

Massive neutrinos also could contribute to dark matter. Dark matter,
like missing mass, is a concept used to explain puzzling astronomical
observations. In observing far-off galaxies, astrophysicists see more
gravitational attraction between nearby galaxies and between inner and
outer parts of individual galaxies than visible objects—like
stars that make up the galaxies—should account for.

Because gravity comes from the attraction between masses, it seems like
some unseen, or missing, mass is adding to the gravitational force. The
mass emits no light, so it is also called dark matter.

Scientists have known for a long time that visible matter is only a
small fraction of the mass of the universe; the rest is a kind of matter
that does not radiate light. Neutrinos with the kind of mass difference
measured by Super-Kamiokande could make up a big part of that dark
matter if their mass is much larger than the tiny splitting between
flavors.

Zukav, Gary.
The Dancing Wu Li Masters: An Overview of the New Physics.
New York: William Morrow, 1979.

NEUTRINOS AND DARK MATTER

According to current theory, the universe is "flat"; in
other words, it contains just the right amount of matter so that it is
at a point exactly in between collapse and infinite expansion. However,
astronomers are only able to observe a fraction of the matter that is
required to make the universe flat and therefore believe that most of
the matter in the universe is unobservable. In fact, the edges of many
galaxies rotate faster than they should if they contained only the
visible matter. The fast motion of galaxies in clusters indicates that
they are gravitationally held together by the presence of mass that
cannot be observed.

According to estimates, about 90% of the mass of the universe consists
of dark matter—matter that does not radiate light and therefore
is invisible to us. A part of the dark matter consists of
"normal," or "baryonic" matter, matter made
up of electrons, protons, and neutrons, the constituents of atoms with
which we are familiar. This matter would be found in the invisible
"brown dwarfs," stars that are very cool and radiate
little, cool intergalactic gas, and the so-called MACHOs (massive
compact halo objects).

A large part of dark matter should also consist of non-baryonic
particles—exotic particles that do not make up normal matter.
Several of these that still would have to be discovered have been
proposed over the years: WIMPs—weakly interacting massive
particles, such as gravitinos, axions, and neutralinos. Up until now
none of these particles has been detected. However, astrophysicists know
that huge quantities of neutrinos were produced during the Big Bang. The
discovery that neutrinos have mass has far-reaching implications for
cosmology. The density of neutrinos is so high—more than 300
neutrinos per cubic centimeter—that even if the mass of neutrinos
is very tiny (millions of times less than that of electrons) they should
account for a sizeable portion of dark matter, perhaps up to 20%.
Because neutrinos move at velocities close to that of light, they would
make up the "hot" dark matter in the universe. Because of
their speed they would "erase" smaller structures, such as
galaxies. The existence of these galaxies is viewed as an argument that
"cold" dark matter, in the form of normal matter or the
slow-moving massive WIMPs, must coexist with hot dark matter.

—Alexander Hellemans

KEY TERMS

ATOM:

Smallest unit of matter that can take part in a chemical reaction and
cannot be broken down chemically into anything simpler.

BETA DECAY:

Transformation of a radioactive nucleus whereby its number of protons is
increased by one through the conversion of an neutron into a proton by
the emission of an electron (and an antineutrino).

CHERENKOV RADIATION:

Named for Soviet physicist Pavel Cherenkov, who discovered this effect
in 1934. It occurs as a bluish light when charged atomic particles pass
through water or other media at a speed greater than the speed of light.

COSMIC RAYS:

Highly energetic particles, mainly electrons and protons, that reach the
earth from all directions.

DARK MATTER:

Any matter in the universe that gives off no light of its own and does
not interact with light the way typical matter does.

EINSTEIN'S MASS-ENERGY EQUIVALENCY PRINCI PLE:

Principle stating that the total energy of a body is equal to its rest
mass times the square of the speed of light.

ELECTRON:

Stable, negatively charged elementary particle that is a constituent of
all atoms and a member of the class of particles called leptons.

FLAVOR:

Property that distinguishes different types of particles. Three flavors
of neutrinos exist: tau, muon, and electron neutrinos.

LAW OF CONSERVATION OF ENERGY:

Law that says energy can be converted from one form to another, but the
total quantity of energy stays the same.

MUON:

Fundamental charged particle (lepton) comparable to the electron in that
it is not constituted of smaller particles, but with a mass
approximately 200 times that of the electron.

NEUTRON:

One of the three main subatomic particles. A composite particle made up
of three quarks. Neutrons have about the same mass as protons, but no
electric charge, and occur in the nuclei of all atoms except hydrogen.

PROTON:

Positively charged elementary particle and a constituent of the nucleus
of all atoms. It belongs to the baryon group of hadrons. Its mass is
almost 1,836 times that of an electron.

RADIOACTIVE DECAY:

Process of continuous disintegration by nuclei of radioactive elements
like radium and isotopes of uranium. This changes an element's
atomic number, turning one element into another, and is accompanied by
emission of radiation.

STANDARD MODEL OF PARTICLE PHYSICS

(not to be confused with the "Standard Model" of the Sun)
:
General theory unifying the electric, weak, and strong force that
predicts the fundamental particles. All predicted have been observed
except for the Higgs particle.

SUPERNOVA:

The explosive death of a star, which temporarily becomes as bright as
100 million or more suns for a few days or weeks. The name
"supernova" was coined by U.S. astronomers Fritz Zwicky
and Walter Baade in 1934.

SYNCHROTRON:

Circular particle accelerator in which the applied magnetic field
increases to keep the orbit radius of the particles constant when their
speed increases.

TAU LEPTON:

Fundamental charged particle comparable to the electron with a mass
about 3500 times that of the electron.

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